This invention relates to the use of modified polyisocyanates as crosslinking agents for binders used in textile printing pastes.
Non-self-crosslinking film-forming binders, for example, those based on polyacrylates or butadiene/acrylonitrile copolymers, are widely used in the textile printing industry. The mixtures of binder and pigment normality contain additionally reactive compounds which, after printing, crosslink the binder at a relatively high temperature and are thus able to fix the print. It is only this fixing that leads to the required water resistance that is important, for example, in washing the printed textiles. (The term "pigments" in the context of the invention also includes dyes.)
In practice, melamine/formaldehyde condensates which enter into crosslinking reactions only at temperatures above about 120.degree. C. are most commonly used as crosslinking agents for the binders used in textile printing pastes. According to W. Berlenbach in Ullmanns Encyklop adie der technischen Chemie, 4th Edition, Vol. 22, page 629, Verlag Chemie, Weinheim 1982, crosslinking is catalyzed by acids via N-methylol groups in the binder. The print is best fixed by exposure to dry hot air, for example, for 5 to 10 minutes at 140.degree. C. or for 30 to 60 seconds at 175.degree. C.
German Offenlegungsschrift 3,529,530 proposes textile printing pastes that contain separately crosslinkable binders dispersed in water or in organic solvents and "deactivated" (i.e. partly blocked) polyisocyanates as crosslinking agents.
German Offenlegungsschrift 3,109,978 describes water-based textile printing pastes that contain film-forming separately crosslinkable polymers as binders and isocyanate prepolymers as crosslinking agents. By comparison with low molecular weight polyisocyanates, these prepolymers apparently provide the printing pastes with a longer pot life by virtue of their higher molecular weights and their lower content of reactive groups.
German Offenlegungsschrift 3,836,030 describes coating compositions that are suitable for the preparation of coatings permeable to water vapor on leather and textiles by the method of evaporation coagulation. In addition to thickeners, these coating compositions contain carboxylate- and/or sulfonate- and/or polyethylene oxide-modified (and hence "hydrophilicized") polyurethanes in aqueous dispersion, high-boiling organic solvents as "non-solvents" for the polyurethanes dispersed in water, NCO-free crosslinking agents, and hydrophilic polyisocyanates to promote coupling with the substrate and as crosslinking agents for the dispersed polyurethanes. During the evaporation process, the non-solvent leaves micropores in the polyurethane film remaining behind and thus guarantees the desired permeability of the coating to water vapor.
German Offenlegungsschrift 3,512,918 relates to hydrophilic isocyanate derivatives containing carbodiimide groups and to their use as crosslinking agents for polymers present in aqueous solution or dispersion that are said to be suitable inter alia for the coating of textiles. Printing pastes are not mentioned.
British Patent 962,109 discloses coating compositions based on special copolymers that are crosslinked with diepoxides. The disclosed compositions may also be used for the preparation of printing pastes. However, the printing pastes in question are not entirely satisfactory in regard to the reactivity and fastness values of prints produced using them.
The disclosed textile printing pastes of the prior art are attended by various disadvantages. Either formaldehyde is given off during the crosslinking reaction and also subsequently in the use of the printed textiles, which is particularly undesirable, or the number of reactive groups available for the crosslinking reaction is difficult to control or the crosslinking agent can be dispersed only with some effort. In addition, known printing pastes generally do not give satisfactory fastness values when heat-treated at temperatures below 100.degree. C. Accordingly, the problem addressed by the present invention was to provide textile printing pastes which would not have any of these disadvantages.
It has now surprisingly been found that this problem can be solved by the use of special mixtures of hydrophilicized polyisocyanates, which give prints having excellent fastness values.